1. Field of the Invention
This invention relates to the separation of carbon from coal ash, more particularly to a froth floatation process for separating carbon from coal ash.
2. Description of the Related Art
Coal ash formed from burning pulverized coal in a furnace usually contains two types of ashes, namely bottom ash and fly ash. The chemical content and particle size of these ashes vary wildly in accordance with the source of coal and the burning conditions of the furnace. The coal ash comprises carbon and mineral materials therein, and normally can be utilized as a mineral admixture for use in the manufacture of Portland cement. However, such usage requires the fly ash to have a carbon content below a level standardized for the Portland cement. The fly ash having a high carbon content may cause undesirable reduction in entrained air in concrete, and exhibits a decrease in desirable pozzolanic reactivity. Such fly ash fails to meet the specification of the Portland cement and can't be reused. Additional cost is necessary for disposal of the fly ash.
As for the bottom ash, although it has similar chemical content as the fly ash, it usually contains larger particle size, higher contents of carbon and coal tar, and has a higher density than the fly ash. The bottom ash normally can't be reused without further treatment, and has to be disposed as waste which will cause disposal problems and environment concerns.
Froth floatation process for the processing of a fly ash has been known in the art. In this process, the carbon of the fly ash is frothed up, and the non-frothed part of the fly ash, which is dominated by mineral materials settls down. Thus, the carbon and the mineral materials which are originally mixed together in the fly ash can be separated. The conventional froth floatation process generally comprises the steps of: adding the fly ash into a mixing tank containing a slurrying liquid to form a slurry; adding a floatation reagent to the slurry to condition the slurry and form the carbon of the fly ash into a hydrophobic carbon; and supplying air bubbles to froth the hydrophobic carbon upwardly to the surface of the slurry, and simultaneously settling the non-frothed part of the fly ash to the bottom of the slurry. The hydrophobic carbon frothing on the surface of the slurry, and the non-frothed part of the fly ash settling on the bottom of the slurry are subsequently removed for recovery. Although the conventional froth floatation process permits removable of the carbon from the fly ash, it has disadvantages in that the carbon can't be efficiently separated from the fly ash, and that the non-frothed part of the fly ash can't have a low carbon level suitable for reuse, for example, in the manufacture of the Portland cement.
The prior art has suggested that the removable of the carbon from the fly ash may be improved by adding an excess amount of the floatation reagent to the slurry in the froth floatation process. However, such a step is impractical due to the increase in the cost of the froth floatation process.